Differential pressure sensors measure a difference in pressure between two isolated fluids or gasses. When used in an environment which includes conductive or corrosive gasses or fluids, the sensor must be isolated from these harsh media in order to protect the sensor itself, as well as the electronic or electrical components attached to the sensor. Differential pressure sensors are harder to isolate from harsh media than gage or absolute pressure sensors due to the two pressure sources being applied to opposing sides of the sensor. Therefore, both sides of the sensor must be isolated in some way or the electronic-pressure sensor device may be damaged.
A differential pressure sensor (or transducer) converts a difference in pressure to an electrical signal that can be measured to determine the differential pressure value. A pressure-sensing device is typically manufactured using micro-machined or Micro-Electro-Mechanical System (MEMS) type methods. This technology is used to manufacture commercial semiconductors along with etching and bonding techniques to fabricate very small, inexpensive devices that convert differential pressure to an electrical signal. The materials used in these devices do not resist corrosion as well as other well known corrosive resistant metals such as stainless steel, titanium, copper and brass, which are typically used in corrosive fluid and gas plumbing. For this reason, an isolation method is required to act as a barrier for corrosion but allow pressure to be communicated to the pressure sensing device without substantially degrading the signal.
The pressure-sensing die is formed from a semiconductor material such as silicon. FIG. 1 is a sectional view of a MEMS type pressure sensing die 100 of the prior art. The die 100 is formed from a silicon wafer by methods such as dicing to produce a silicon structure 101. The structure 101 is thinned to create a cavity 105 and a thinned portion defining a diaphragm 103. The semiconductor structure 101 may be thinned by any suitable means, for example, the structure 101 may be thinned using anisotropic etching as known in the art. Resistive elements are formed on the surface of the diaphragm 103. The resistive elements exhibit resistance that is proportional to the strain placed on the thinned semiconductor material forming the diaphragm 103.
FIG. 2 is an illustration of a prior art MEMS pressure sensor designed as a gage or absolute pressure measurement device using pressure sensing die 100. Pressure sensing device 100 is typically mounted to a support structure 207 which is, in turn bonded to a base plate 201, formed from a non-corroding material, for example, stainless steel. The sensing die 100 and the support structure 207 may be bonded to base plate 201, which may also be termed a header, by an adhesive 205. The support structure 207 is used as it isolates the pressure sensing device 100 from sources of strain that are unrelated to pressure, such as thermal expansion which varies between the pressure sensing device 100 and the base plate 201. An opening 203 is defined in the base plate 201 defining an aperture which is in fluid communication with the underside of the diaphragm of pressure sensing device 100. The opening 203 allows ambient pressure to come in contact with one side of the pressure sensing device 100 providing a reference pressure. The reference pressure may used in measuring the pressure of a fluid under test which exerts pressure on the opposite side of the pressure sensing die 100. The pressure sensing die 100 is attached to the base plate 201 over the opening 203 via support structure 207. Support structure 207 may be formed from glass or similar material which has a coefficient of thermal expansion closer to that of the silicon pressure sensing die 100 as compared to the coefficient of thermal expansion of the stainless steel making up the base plate 201. This matching of the coefficients of thermal expansion prevents exertion of forces on the die 100 not related to pressure, but rather, caused by the strain related to the dissimilar rates of expansion between the die 100 and the base plate 201. The constraint 207 is attached to the base plate 201 by an appropriate adhesive 205 as known in the art. For example, bonding may be performed by a Silicone adhesive, epoxy, solder, braze or other commonly known techniques.
The pressure sensing device 200 includes upper housing 223. Upper housing 223 is configured to provide a sealed attachment to base plate 201. An enclosed volume is defined between upper housing 223 and base plate 201. Flexible corrugated diaphragm 221 serves to divide the enclosed volume into a first volume 219 and a second volume 227. Port 225 is defined through a wall of upper housing 223 and in communication with first volume 219. Port 225 may be coupled to a fluid source which is to be tested for pressure. Pressure sensing die 100 further includes electrical components which create and transmit an electrical signal indicative of a pressure exerted on the die 100. In applications where the fluid being tested is a harsh medium, such as fuel or oil, such media may corrode the electrical components of the die 100. Therefore, care must be taken to isolate the die 100 from the fluid being tested. Isolation is accomplished by flexible corrugated diaphragm 221. An oil fill port 215 is provided through the base plate 201. The oil fill port allows the volume 219 between the die 100 and the diaphragm 221 to be filled with a non-corrosive fluid such as silicone oil. When the cavity defining volume 219 is filled, the oil fill port 215 is sealed, for example, by welding a ball 217 across the opening of the oil fill port 215. The oil in volume 219 is thus fully enclosed and in fluid communication with the upper surface of die 100.
Port 225 may be threaded to allow the pressure sensing device 200 to be attached to a line or other transmission means in fluid communication with the fluid to be tested or measured. The fluid being measured enters the port 225 and fills the interior volume 227. When the interior volume 227 is filled, the fluid being measured is in contact with the upper side of the flexible diaphragm 221. Pressure exerted by the fluid being measured is transmitted through the flexible diaphragm 221 to the enclosed volume 219 of oil. The force applied to the oil by the flexible diaphragm 221 is transmitted throughout the oil and to the surfaces containing the oil, including the upper surface of pressure sensing die 100.
When a force is exerted on pressure sensing die 100, an electrical signal through piezo-resistive elements formed in the upper surface of the diaphragm of pressure sensing die 100 varies responsive to variations in the piezo-resistive elements. The electrical signal is representative of the force applied to the surface of the pressure sensing die 100. The electrical signal is conducted via bond wires 209 to conductive pins 211 which may be electrically connected to other system circuitry, such as a control circuit, or converted to pressure data which may be stored, by way of non-limiting example, in an electronic memory.
The flexible diaphragm 221 and oil filled volume 219 isolate the die 100, bond wires 209 and conductive pins 211 from the corrosive or harsh media being measured via port 225. Additionally, the volume 219 containing the oil must be sealed such that leakage or contamination of the oil within volume 219 does not occur. Conductive pins 211 carrying the electrical signal from the pressure sensing die 100 must pass through the base plate 201 to allow external connection of other system components. Conductive pins 211 are enclosed in a glass or ceramic material fired into a tube or hole 213 which forms a hermetic seal with base plate 201. Hermetic seals are expensive to produce and fragile, but are necessary to ensure the integrity of the volume 219. A pressure sensor which provides isolation of the sensing components and associated circuitry from harsh media being measured in a simple and inexpensive form factor is therefore desired.